Chemotaxis is directional movement in response to a specific chemical gradient. This cellular ability is necessary for immune homeostasis and the response to inflammation, among other critical biologic processes. Several chemokines have been identified along with their receptors, providing a molecular mechanism to orchestrate movement of distinct cell types in response to diverse stimuli. For example, chemokine receptor 7 (CCR7) and its ligands, CCL19 and CCL21, comprise a signaling axis required for chemotaxis of T-cells into and within lymphoid organs. CCR7-mediated chemotaxis is important in developing adaptive immunity, as well as maintaining tolerance and memory.
Chemokines are broadly grouped as homeostatic or inflammatory. For the latter, acutely increasing production may be sufficient to control a chemotactic response. For homeostatic chemokines, such as CCL19/21, signal modulation occurs by altering receptor density or effective ligand concentration. This is achieved either directly (e.g. increased receptor expression) or indirectly (e.g. atypical chemokine receptor scavenging of ligands). Indeed, for CCR7, exposure to serum promotes cell migration, and there is an enhanced chemotactic response of T-cells to CCL19/21 in the presence of serum, although the basis for this acceleration has not been previously described in the literature.